1. Field of the Invention
The present invention relates generally to blast nozzles used to remove adherent material, such as paint, scale, dirt, grease and the like from solid surfaces with abrasive particles propelled by air. In particular, the present invention is directed to an improved blast nozzle apparatus which provides a uniform flow of air and blast media to the inlet of the blast nozzle and consequently produces a uniform flow of blast media from the nozzle outlet.
2. Description of the Prior Art
In order to clean a solid surface so that such surface can again be coated such as, for example, to preserve metal against deterioration, or simply to degrease a solid surface such as surfaces contacting food or building structures which contain food serving or food processing operations, it has become common practice to use an abrasive blasting technique wherein abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminates. Various abrasive blasting techniques have been utilized to remove the coatings, grease and the like from solid surfaces. Thus, blasting techniques comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, multi-step processes comprising dry or wet blasting and a mechanical technique such as sanding, chipping, etc. and a single step process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used. Water for dust control has been mixed with the air either internally in the blast nozzle or exterior of the nozzle at the targeted surface to be cleaned and such latter process, although primarily a dry blasting technique, is considered wet blasting inasmuch as media recovery and clean up is substantially different from that utilized in a purely dry blasting operation.
A typical dry blasting apparatus as well as a wet blasting apparatus which utilizes highly pressurized air to entrain, carry and direct the abrasive blast media to the solid surface to be treated and low pressure water for dust control comprises a dispensing portion in which the blast media typically contained in a storage tank is entrained in highly pressurized air, a flexible hose which carries the air/blast media mixture to the blast nozzle and which allows the operator to move the blast nozzle relative to the surface to be cleaned and the blast nozzle which accelerates the abrasive blast media and directs same into contact with the surface to be treated. Water may be added either internally in the blast nozzle and mixed with the air stream passing therethrough or a low pressure stream of water may be provided externally of the blast nozzle and directed at the surface to be treated so as to control dust. The blast nozzle is typically hand-held by the operator and moved relative to the targeted surface so as to direct the abrasive blast media across the entire surface to be treated.
The blast media or abrasive particles most widely used for blasting surfaces to remove adherent material therefrom is sand. Sand is a hard abrasive which is very useful in removing adherent materials such as paint, scale and other materials from hard metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technique, there are disadvantages in using sand as a blast media. For one, sand, i.e., silica, is friable and upon hitting a metal surface will break into minute particles which are small enough to enter the lungs. These minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus, sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other relatively soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.
An alternative to non-soluble blast media such as sand, in particular, for removing adherent coatings from relatively soft substrates such as softer metals as aluminum, composite surfaces, plastics, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from aluminum surfaces and as well remove other coatings including paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water soluble such that the particles which remain subsequent to blasting can be simply washed away without yielding environmental harm. However, sodium bicarbonate is also friable and its lighter weight relative to sand requires operating conditions and apparatus for blasting which may be quite different from the conditions and apparatus used to blast with sand.
In most any abrasive blasting operation, the flexible hose which transports the blast media and pressurized fluid stream from the dispensing device to the blast nozzle often bends at intermediate points and typically is draped over the shoulder of the operator. What has been found is that the flow of abrasive particles or blast media through the nozzle is not uniform inasmuch as the sharp bend formed as the hose travels over the shoulder of the operator results in centrifugal forces which tend to concentrate the blast media to the outside of the bend and, thus, in only one quadrant of the hose. Consequently, the abrasive blast media concentrates in only a portion of the inlet and outlet of the blast nozzle. The performance of the blasting operation is hindered inasmuch as the blast media is not uniform throughout the whole of the blast nozzle outlet causing a reduction of the "hot spot" which is the area of maximum contact of the blast media on the surface being treated at a given moment. A reduction in the hot spot translates directly to a reduction in productivity which can be defined as the volume of contaminant removed from the substrate per time and flow rate of blast media through the nozzle. There may also occur nonuniform erosion of the interior of the blast nozzle and the supply hose near the blast nozzle inlet in view of the concentrated abrasive flow. Erosion and wear within the blast nozzle may result in turbulent flow of the abrasive blast particles through the nozzle which reduces the velocity of the particles and consequently also reduces productivity.
Accordingly, it would be worthwhile to provide a uniform flow of the abrasive blast media into the inlet of the blast nozzle to prevent turbulent flow of the media particles and obtain maximum velocity of media from the nozzle outlet and, as well, provide uniform flow through the whole of the nozzle outlet to maximize the hot spot, thereby providing optimum production rates.